This invention relates to a method and apparatus utilizing digital logic for minimizing visual perception of the raster lines on a photographic film recording of a video or TV monitor picture. The principles of operation of a raster erase method utilizing analog rather than digital components, are disclosed in U.S. Pat. No. 4,096,530 issued to J. S. Plugge, William H. Wesbey, and James E. Blake and assigned to the assignee of the present invention. The disclosure of U.S. Pat. No. 4,096,530 is incorporated herein by reference.
As is well known, when a viewer is closely viewing a cathode ray tube video monitor, the horizontal raster lines can be seen on the face of the picture tube. However, if the viewer is far enough away from the face of the tube, the horizontal lines can no longer be resolved by the eyes, and the picture does not appear to be composed of a multiplicity of horizontal lines.
The present invention is concerned with "erasing" or obscuring the perception of the horizontal raster lines in a photograph taken of the screen of a video monitor tube when a static video picture is being displayed thereby. This results in a photograph that is of a more pleasing quality to human perception. An example of photographically recording static pictures on a video monitor tube arises in the medical field where video monitors are used to display "converted" X-ray images. In X-ray diagnostic systems, an X-ray images is converted to an optical image with an image intensifier, and the optical image is viewed with a video camera. The resulting video signals may be stored in a video recorder, and the signals from the recorder may be used to drive a video monitor. A photographic camera is directed towards the picture tube faceplate to enable photographic recording of any static image that is derived from the recorder and displayed on the monitor. When a conventional video system without a raster obscuration function is used, the raster scan lines are also recorded on the film with such high definition as to be very distracting to anyone studying the photograph for detailed information.
One method that has been proposed for reducing visualization of the raster lines is "dithering" or oscillating the scanning beam over the vertical distance of one pair of original scan lines as the scanning beam moves horizontally across the picture tube. This requires applying a high-frequency signal to the scanning coil of the video display tube such that the space ordinarily occurring between the scan lines appears filled and thusly perception of scan lines is reduced. However, this method has substantial technical problems associated with it as, for instance, the difficulty in controlling beam dithering with precision.
Additionally, it is well known in the TV art that a phenomenon known as "roll" occurs where the horizontal sync pulses are correct and the vertical scanning frequency (vertical sync pulse frequency) is off the normal 60 cps. rate. Under these conditions, the screen shows the bottom part of the televised subject at the top of the screen, the top part of the subject at the bottom of the screen, and the retrace bar as a visible black bar between the top and bottom parts of the screen. The picture continues to "roll" with increasing speed as the frequency of vertical sync pulses increases from the normal 60 cps. At the normal frequency, successive frames overlap. Thus, it can be seen that, if the vertical sync pulse is delayed in a next frame by a slight amount, such next frame appears to move up from the screen position of the original image. This is described in the art as "vertical roll" as described above wherein a large horizontal black bar appears across the screen and separates the bottom of the original image (appearing at the top of the "rolled" screen) from the top of the original image (appearing at the bottom of the "rolled" screen), as the image appears to roll vertically upward. The black bar demonstrates that a normally-transmitted vertical sync pulse is considered part of the image information data such as the black-grey-white image information.
Thus, as the amount of time from the sync pulse to the non-blanked informational horizontal sync pulses decreases, the image appears to shift upwards. In the present invention and in the invention of U.S. Pat. No. 4,096,530, the roll concept is utilized effectively to delay the vertical sync pulse by a predetermined small fraction of the time that normally occurs between the original vertical sync pulses. The original frame is rescanned a predetermined number of times. During each of these rescannings, the corresponding vertical sync pulse is delayed by an incremental amount that is larger than the delay that produced the previous vertical sync pulse. Thus, each frame will be slightly upwardly vertically offset from the previous one, the sum of such offsets being less than the distance between the original horizontal raster lines. When photographed, the raster lines associated with the original frame will appear to be obscured. Rescanning is repetitively performed over a predetermined number of incrementally and vertically-spaced positions, but short of coinciding with the next normal information line.
Therefore, picture information from adjacent lines of the orignal image is blended into the space between the adjacent horizontal lines of the original image during a photographic exposure interval.
For example, in the invention of U.S. Pat. No. 4,096,430, 16 frames occur, with the first being normal, and the following 15 frames each being offset from the previous frame by a vertical space equal to 1/16 of the distance between original or normally-spaced horizontal scan lines. With these rescannings, visual perception of obscuration of the raster lines results. In an interlaced system, the delay used for the rescanning of a frame is actually the use of the same delay time for each of both the odd and even fields for each of the rescannings.
Additionally, in the system of U.S. Pat. No. 4,096,530, a photographic camera is incorporated, and an exposure button is closed each time a photograph of a video image is to be taken. The shutter of the camera is electromechanically connected to and opened by the raster erase system. Thereafter, sixteen scannings of the static picture is displayed, and the camera shutter is subsequently electrically closed, following which a film transport is energized to move the film to the next photographic-negative position. In that system, both analog and digital control circuitry are used. In such type of system, however, it would be desirable to obviate the need for an integrated camera and thereby reduce the complexity and cost of the system. In addition, it would be desirable to increase electronic reliability and decrease component cost by utilizing all-digital electronic circuitry.